306 ANNUAL REPORT SMITHSONIAN INSTITUTION, 192 8 



The craters resemble in form those of the Hawaiian Islands. 

 They appear to have been opened by gaseous drilling from below, as 

 are terrestrial craters, and by final explosive outrushes of gas rising 

 through pipe,s filled with lava or breccia, but for some reason the 

 igneous forces in the moon lack the power to produce extensive over- 

 flows. On the other hand, the alignment of small craters, as those on 

 the left of Copernicus, shows that they are not, as some have sur- 

 mised, the results of impact from the outside during the original 

 formation of the moon. 



Although craters of all sizes may be noted in the moon from the 

 minute craterlets to the great maria, the latter appear to represent a 

 different kind of activity. Within the Mare Imbrium there are 

 remnants of a ring outlining an older and smaller feature. The 

 older, rough topography of the floor of the mare flattens and fades 

 out by degrees. To understand this feature, imagine a rough sur- 

 face of pitch, with an outer temperature far below 0° F., heated from 

 below until the interior became fluid, the outer skin remaining cold 

 and solid, but the supporting layer passing into a soft and viscous 

 state. The relief would therefore flatten down, but not be absolutely 

 lost. Such appears to be the most instructive analogy for the inter- 

 pretation of the surfaces of the maria, although in places thinly fluid 

 lava may have overflowed. 



The craters probably are gas vents, and are instructive, by analogy, 

 in regard to the continued supply and internal origin of the terrestrial 

 atmosphere and hydrosphere. The maria seem to represent the 

 culminating rise of magma from the depths into the outer crust at 

 long intervals of time, accompanied by a melting in of the covers, and 

 correspond to the broad batholiths exposed on the earth by erosion. 



We are now prepared to compare instructively the maria of the 

 moon with the ocean basins of the earth. The great lava plain on 

 the east side of the moon corresponds rudely to the Atlantic Ocean ; 

 the embayments on the south may be likened in form to the Gulf of 

 Mexico and the Caribbean Sea; the coalescent maria on the west, 

 Serenitatis, Tranquilitatis, Fecunditatis, and Nectaris, to the chain 

 of Mediterranean deeps ; the isolated Mare Crisium in the northwest 

 to the similarly isolated basin of the Black Sea. The uplands occupy 

 the lesser portion of the surface, extend into the angles between the 

 maria, and in places are seen to fault olf and subside into them. 

 These resemblances to earth characters also caught the eye of Galileo 

 and led him to regard the lava floors as in reality lunar seas. 



Thus the study of the earth's small sister planet supports the gen- 

 eral hypotheses that the ocean waters as well as the ocean basins have 

 arisen through igneous activity, and that fragmentation of the 

 original crust has dominated the moon as well as the earth through- 

 out geological time. 



